Typical examples of a laminated optical disk include DVD (digital versatile disk or digital video disk). This DVD is produced by a method of at least laminating two substrates for optical disk, the substrate being obtained by forming an information recording layer on one substrate for optical disk. A laminating agent used in case of lamination is generally an adhesive comprising an ultraviolet curable composition.
The information recording layer is a laminate comprising a substrate for optical disk made of a synthetic resin such as polycarbonate; a layer comprising irregularities referred to as pits, a phase-change material, a pigment and the like, formed on the substrate, and a semi-reflective or complete reflective layer for reflecting laser beams for information reading, formed thereon. The semi-reflective layer and the complete reflective layer are layers which are formed on the top of the information recording layer and are generally composed of a thin film of a metal or a metal alloy.
In a reproducing-only type DVD having a structure in which two substrates for optical disk are laminated, various optical disks exist. For example, an optical disk referred to as “DVD-10” is produced by preparing two polycarbonate substrates for optical disk wherein irregularities corresponding to recording information referred to as pits are provided on one surface of a substrate, and then an aluminum layer is formed as a layer for reflecting laser beams for information reading thereon, and laminating them using the aluminum layer as an adhesive surface. “DVD-5” is produced by laminating the above polycarbonate substrate for producing “DVD-10” with a conventional transparent polycarbonate substrate with no information recording layer formed thereon. “DVD-9” is produced by laminating a substrate, which is obtained by forming an aluminum reflective layer on pits provided on one surface of a substrate, with a substrate, which is obtained by forming a semi-reflective layer made of gold or an alloy containing gold as a main component, silver or an alloy containing silver as a main component, or a silicon compound on pits provided on one surface of a substrate, using the reflective layers as an adhesive surface. Furthermore, “DVD-18” is produced by laminating two substrates each comprising two information recording layers formed on one surface. “DVD-9”, which has a large recording capacity and is capable of reading information of two layers from one surface, is currently prevailing.
As the material of a semi-reflective layer of DVD-9 or the like, gold or a silicon compound is mainly used. However, gold is very expensive and is therefore disadvantageous in view of cost and the silicon compound has a drawback that a film formation is very difficult. Therefore, a study on replacement by silver or an alloy containing silver as a main component has intensively been made because it is cheap as compared with gold and a film is formed with ease.
However, there arose a problem that durability of DVD-9 comprising silver or an alloy containing silver as a main component as a semi-reflective layer drastically deteriorates, when a conventional ultraviolet curable adhesive which is used in the production of DVD-9 comprising gold or a silicon compound as a semi-reflective layer is used. Specifically, when DVD-9, which uses a silver or an alloy containing silver as a main component in a semi-reflective layer and uses a conventional adhesive, is exposed in a high-temperature and high-humidity atmosphere for a long time, deterioration of the surface of silver or an alloy containing silver as a main component is caused by an adverse influence of the adhesive and an increase in number of errors of reading signals and poor appearance occur, and thus durability of DVD-9 drastically deteriorates.
To solve the above problems in a high-temperature and high-humidity atmosphere, for example, there is proposed a technique in which an ultraviolet curable composition containing phenylthioethyl (meth)acrylates is used as an adhesive of DVD-9 (see, for example, claims and examples of Japanese Unexamined Patent Publication (Kokai) No. 2002-212514, hereinafter referred to as Patent Document 1). In Patent Document 1, there is described that, even in case of DVD-9 in which silver or an alloy containing silver as a main component is used as a semi-reflective layer, after conducting a test in a high-temperature and high-humidity atmosphere at 80° C. and 85%RH for 500 hours, neither discoloration of the semi-reflective layer nor formation of pinholes occur and the same durability as that of DVD-9 in which gold is used as the semi-reflective layer can be obtained.
Another problem in an optical disk comprising a metal reflective layer such as DVD-9 is corrosion of the metal reflective layer due to an ultraviolet curable composition used as an adhesive. To solve the problem, for example, there is reported a technique in which an adhesive composition containing a cationic polymerizable compound, a photocationic polymerization initiator, and hydroxycarboxylic acid as an aluminum corrosion inhibitor is coated onto a thin film such as aluminum deposited film (see, for example, claims, column 6 and column 25 of Japanese Unexamined Patent Publication (Kokai) No. 2002-146331, hereinafter referred to as Patent Document 2). Examples of the hydroxycarboxylic acid include citric acid, isocitric acid, tartaric acid, malic acid, lactic acid, 2-methyllactic acid, hydroxypyruvic acid, dimethylolbutanoic acid, dimethylolpropionic acid, α-hydroxyisobutyric acid, α,α-diphenylglycolic acid, mandelic acid, 6-hydroxy-2-naphthoic acid, p-hydroxybenzoic acid, 5-hydroxyisophthalic acid, p-hydroxyphenylacetic acid, p-hydroxyphenylpropionic acid, citrazinic acid, chelidamic acid and gallic acid.
In the optical disk comprising a metal reflective layer, adhesion to the metal reflective layer of the ultraviolet curable composition used as the adhesive is also an important problem. For example, there is proposed a technique relating to a composition having excellent adhesion to both an amorphous silicone resin substrate and a metal reflective layer made of gold, silver, copper, aluminum or the like (see, for example, claims, column 28, column 29 and column 34 of Japanese Unexamined Patent Publication (Kokai) No. 2002-285042, hereinafter referred to as Patent Document 3). In this technique, a technique relating to an ultraviolet curable composition for optical disk, containing dicyclopentadiene diacrylate, a thixanthone-based compound and (meth)acrylate having a phosphoric acid is proposed. There is also described that polymerization inhibitors such as hydroquinone monomethyl ether, t-butyl catechol, p-benzoquinone, 2,5-t-butyl-hydroquinone and phenothiazine can be used in an amount within a range from 0.1 to 5% by mass based on the total amounts of the ultraviolet curable composition for optical disk.
By the way, there is described a technique regarding a rapid curing two-component type acrylic adhesive which is excellent in adhesion in which an adhesive composition which contains an acrylic compound selected from (meth)acrylic acid and esters thereof, a peroxyester-based polymerization initiator, a gallic acid derivative and other additives is used as a first component solution (see, for example, claims, page 6, upper right column, line 5 to the same page, lower left column, line 17, and examples of Japanese Unexamined Patent Publication (Kokai) No. 3-134081, hereinafter referred to as Patent Document 4). There is also proposed that, in order to cure a first component solution squeezed out of the bonding portion upon bonding, a photopolymerization initiator is added to an adhesive composition and the squeezed-out adhesive is cured by irradiating with ultraviolet light.
As described above, in the technical field of an optical disk, improvements in durability when exposed in a high-temperature and high-humidity atmosphere for a long period, corrosion resistance of a metal reflective layer, adhesion to a metal reflective layer of an ultraviolet curable composition or the like have hitherto been studied. The present inventors have intensively studied about the development of an optical disk comprising a reflective layer made of silver or an alloy containing silver as a main component, and also studied these conventionally known problems. As a result, they have found that additional problem exists, in addition to the above problems. The problem is a problem in which, when an optical film comprising a reflective layer made of silver or an alloy containing silver as a main component is exposed to room light such as fluorescent lamp, this reflective layer is blackened thereby to cause a decrease in reflectance and an increase in PI error (parity of inner-code error), and thus it becomes impossible to read information in the worst case. It is very important to solve this problem so as to improve practical characteristics of an optical disk comprising a reflective layer made of silver or an alloy containing silver as a main component.
Thus, an object of the present invention is to provide an optical disk comprising a reflective layer made of silver or an alloy containing silver as a main component, having excellent light resistance, which does not cause blackening of a reflective layer and also causes neither an increase in read error (PI error) of signals nor a decrease in reflectance even when exposed to room light such as fluorescent lamp.